A back-illuminated image sensor is devised in which a light receiving surface of a photodiode is provided on the rear surface of a semiconductor substrate contrasted with a front-illuminated image sensor in the past in which light is made incident from the front surface of a semiconductor substrate. In the back-illuminated image sensor, because it is unnecessary to form wires and unnecessary films on the light receiving surface, sensitivity higher than that of the front-illuminated image sensor can be obtained.
In the back-illuminated image sensor, it is necessary to reduce the semiconductor substrate in thickness to efficiently collect, on a photodiode, light made incident on the rear surface of the semiconductor substrate. The thickness of the semiconductor substrate needs to be set to thickness for preventing resolution from being deteriorated until charges generated on the light receiving surface are diffused and collected on the photodiode. For example, when visible light is made incident, the thickness needs to be set to be smaller than 20 micrometers.
Such a back-illuminated image sensor is formed by, for example, a method explained below. First, a semiconductor substrate having a photodiode and an integrated circuit formed on the front surface thereof is prepared. A supporting substrate having a diameter substantially the same as that of the semiconductor substrate is joined to the front surface side of the semiconductor substrate. The semiconductor substrate is reduced in thickness from the rear surface side to near the photodiode to form a light-receiving surface on the rear surface of the semiconductor substrate. The supporting substrate functions as a reinforcing member when the semiconductor substrate is reduced in thickness. A reflection preventing film, a color filter, a condensing micro-lens, and the like are provided on the light receiving surface.
Further, an electrode section electrically connected to the integrated circuit on the front surface is formed on the rear surface of the semiconductor substrate. Thereafter, a joined member of the semiconductor substrate and the supporting substrate are cut and divided by a dicing blade. Divided chips are bonded to a ceramic package or the like and electrode sections of the chips and wires formed on the ceramic package are electrically connected by wire bonding to obtain a semiconductor device. In this way, the semiconductor substrate having a function of a so-called back-illuminated image sensor is formed.
In the semiconductor device, the semiconductor device is reduced in thickness from the rear surface of the semiconductor device to a layer on the front surface in which the photodiode is formed. When the semiconductor device is reduced in thickness, the semiconductor substrate is reduced in thickness halfway by mechanical grinding or chemical mechanical polishing. To efficiently collect energy beams on the photodiode, the semiconductor substrate is desirably formed as thin as possible. However, because the semiconductor substrate is reduced in thickness, when the integrated circuit (including metal wires and insulative film) is formed on the front surface of the semiconductor substrate, residual stress is concentrated on a joint surface side of the semiconductor substrate and the supporting substrate. Therefore, the joining of the semiconductor substrate and the supporting substrate is desirably performed by a joining method for reducing the influence of the residual stress. Because a high-temperature process is necessary in forming an electrode on the rear surface of the semiconductor substrate, the method of joining the semiconductor substrate and the supporting substrate is desirably a method of joining the semiconductor substrate and the supporting substrate not via an organic material. Consequently, the method of joining the semiconductor substrate and the supporting substrate is desirably a direct joining system for directly connecting the front surface section of the semiconductor substrate and the front surface section of the supporting substrate wirelessly (see, for example, the specification of U.S. Pat. No. 7,479,441).
In the direct joining system, a joining starting point is formed by pressing a predetermined one point. A joint interface spontaneously expands from the joining starting point in an isotropic manner. However, a joining property of the direct joining substantially depends on a state of the substrate front surface. Therefore, when flatness and cleanness of the front surface are deteriorated, expansion speed of the joint interface decreases, the isotropic expansion of the joint interface is deteriorated, an air layer is entrained in the joint interface. As a result, a void occurs, the expansion of the joint interface is stopped halfway, and an un-joined section is formed. When the un-joined section is formed, when the semiconductor substrate is reduced in thickness, separation of the semiconductor substrate and the supporting substrate, a break of the thin semiconductor substrate, or the like occurs, and the yield of generation of the semiconductor device decreases. When the separation or the break does not occur, if a void is present, the thin semiconductor substrate is deformed, the light receiving surface is distorted, and an imaging characteristic is deteriorated.